1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to an apparatus and a method for reverse/forward ACKnowledgement/NonACKnowledgement (ACK/NACK) transmission, which can support Hybrid Automatic Retransmission reQuest (HARQ) for multiple reverse/forward layer data transmissions in an Orthogonal Frequency Division Multiplexing (OFDM) based mobile communication system.
2. Description of the Related Art
In general, HARQ is an important technique used to improve data throughput and reliability of data transmission in a packet-based mobile communication system. The HARQ corresponds to a combination of the techniques of Automatic Retransmission reQuest (ARQ) and Forward Error Correction (FEC).
According to the ARQ technique, which is being widely used in wire/wireless data communication systems, a transmitter transmits data packets with sequence numbers attached to the data packets according to a set scheme, and a receiver requests retransmission of a missing packet from among the received packets by using the sequence numbers, thereby achieving reliable data transmission.
According to the FEC, each data packet is transmitted together with a redundant bit added thereto according to a rule, such as convolutional encoding or turbo encoding, so that the originally-transmitted data can be demodulated without noise or fading, which may occur during the data transmission/reception. In a system using the HARQ, that is, a combination of the aforementioned two techniques, the receiver performs a Cyclic Redundancy Check (CRC) for data demodulated through an inverse FEC process, in order to determine whether the data has an error. As a result of the CRC, when the data has no error, the system using the HARQ feeds back an ACKnowledgement (ACK) to the transmitter, so that the transmitter transmits a next data packet. However, when the CRC check shows that the data has an error, the HARQ system feeds back a Non-ACKnowledgement (NACK) to the transmitter, so that the transmitter retransmits the previously transmitted data packet. During this process, the receiver obtains an energy gain by combining the retransmitted packet with the previously-transmitted packet. Therefore, the HARQ system can achieve a significantly improved performance in comparison with a typical ARQ system that does not perform such a combining process.
FIG. 1 illustrates an example of a conventional HARQ.
In FIG. 1, the horizontal axis corresponds to a time axis, reference numeral 101 denotes initial transmission, and the data channel refers to a channel through which data is actually transmitted. A receiver having received the data transmitted by the initial transmission 101 demodulates the data channel. In the demodulation, when the receiver determines, by the CRC, that the transmitted data has not been correctly demodulated, the receiver feeds back an NACK 102 to the transmitter. Upon receiving the NACK 102, a data channel transmitter retransmits the data of the initial transmission 101 (first retransmission 103). Therefore, it should be noted that the data channels of the initial transmission 101 and the first retransmission 103 carry the same information. Also, it should be noted that the two data channels may have different redundancies, even when they carry the same information. As used herein, each of the data channels carrying the same information, i.e., the data channels of the transmission designated by reference numeral 101, 103, or 105, is called a sub-packet. Upon receiving the data transmitted by the first retransmission 103, the receiver combines the data of the first retransmission 103 with the data of the initial transmission 101 according to a rule and then demodulates the data channel by using the combined data. Through this process, when the receiver determines, by CRC for the data channel, that the transmitted data has not been correctly demodulated, the receiver feeds back a NACK 104 to the transmitter. Upon receiving the NACK 104, the transmitter performs second retransmission 105 of the data after a time interval passes from the time point of the first retransmission 103. As a result, the data channels of the initial transmission 101, the first retransmission 103 and the second retransmission 105 carry the same information. Upon receiving the data transmitted by the second retransmission 105, the receiver combines the data of the initial transmission 101, the first retransmission 103 and the second retransmission 105 with each other according to a rule and then demodulates the data channel by using the combined data. Through this process, when the receiver determines, by CRC for the data channel, that the transmitted data has been correctly demodulated, the receiver feeds back an ACK 106 to the transmitter. Upon receiving the ACK 106, the transmitter transmits an initially transmitted sub-packet 107 for next data information. The initial transmission 107 may be performed either instantly as soon as the ACK 106 is received or after the passage of a time interval, according to a scheduling scheme.
In order to support HARQ as described above, the receiver should feed back the ACK/NACK. As used herein, a channel transmitting the ACK/NACK is called an “ACK CHannel (ACKCH).”
Meanwhile, multiple antenna technology for improving the data rate or throughput of a system includes schemes of Spatial Multiplexing (SM) and Spatial Domain Multiple Access (SDMA). According to the SM, a transmitter transmits multiple data streams through multiple antennas to a single receiver. According to the SDMA, a transmitter transmits multiple data streams to multiple receivers. The SM and SDMA schemes are called schemes using multiple layers.
In order to perform data transmission for multiple layers and support HARQ for each layer, it is necessary to consider an efficient ACKCH. Hereinafter, a conventional method for ACKCH transmission in a system performing data transmission for multiple layers and supporting HARQ for each layer will be described.
First, a transmission method and a resource allocation method for an ACKCH for one layer in a conventional Orthogonal Frequency Division Multiple Access (OFDMA) system are discussed.
In a conventional forward OFDMA data system, a basic resource unit for forward data transmission is defined. As used herein, the basic resource unit is referred to as a “data resource channel.” Usually, one data resource channel is defined for multiple OFDM symbols in the time domain and for multiple sub-carriers in the frequency domain. For example, one data channel includes 8 OFDM symbols and 16 sub-carriers. If a system includes 480 sub-carriers available in the frequency domain, and if the system includes 30 data resource channels, a maximum of 30 bits are necessary for reverse ACK/NACK transmission. This is because it should be possible to transmit one bit of reverse ACK/NACK feedback for each forward data resource channel, in order to enable transmission of the same number of reverse ACK/NACK responses as the number of forward data resource channels. Hereinafter, specific examples of the reverse ACK/NACK transmission of resource allocation for the reverse ACK/NACK transmission according to the prior art will be discussed.
FIG. 2 illustrates a structure of a conventional transmitter for transmitting a reverse ACK/NACK response in response to forward data transmission received by a Mobile Station (MS).
The transmitter includes a first zero inserter 202, a Discrete Fourier Transform (DFT) unit 203, a sub-carrier mapper 204, a second zero inserter 205, an Inverse Fast Fourier Transform (IFFT) unit 206, a Parallel-to-Serial (P/S) converter 207, a Cyclic Prefix (CP) adder 208 and a controller 210.
In FIG. 2, reference numeral 201 denotes ACK/NACK, which is selectively determined according to whether the received forward data has been correctly demodulated, and if not, requires retransmission thereof. The ACK/NACK is input to the 16 point DFT unit 203. From among the DFT inputs, only one input is mapped to a resource channel located at a position corresponding to the resource channel through which the MS has received forward data, while the other inputs to the DFT unit 203 are filled with “0” by the first zero inserter 202. For example, if the data received by the MS in the forward direction has been transmitted through a resource channel “0,” and if the 0th input of the 16 point DFT unit has been mapped in advance to the resource channel “0”, the MS transmits the ACK/NACK by using only the 0th DFT input, while the other inputs of the 16 point DFT unit 203 are filled with “0.” This process is controlled by the controller 210. The outputs of the 16 point DFT unit 203 are input to the sub-carrier mapper 204 for sub-carrier mapping. Specifically, the outputs of the 16 point DFT unit 203 are mapped to sub-carriers at positions from among the 480 sub-carriers as in the above-described example. If the OFDM system employs 512 size Fast Fourier Transform (FFT), the second zero inserter fills “0” in sub-carriers at positions other than those of the outputs of the sub-carrier mapper 204. Then, the sub-carriers are processed by the IFFT unit 206, a P/S converter 207 and a CP adder 208 according to a conventional OFDM symbol generating process for transmission.
FIG. 3 illustrates a conventional mapping relation between reverse ACK/NACK transmission and forward resource channels, and a sub-carrier mapping process by the sub-carrier mapper of FIG. 2.
The outputs of the 16 point DFT unit 203 of FIG. 2 have 16 point values, which are transmitted by the portion designated by reference numeral 310 in FIG. 3. The horizontal axis in FIG. 3 corresponds to a time axis and each segment of the horizontal axis corresponds to one OFDM symbol interval. Further, the vertical axis corresponds to a frequency axis, and each segment of the vertical axis corresponds to one sub-carrier. The entire rectangular box shown in FIG. 3 is called a “tile,” and serves as a basic resource allocation unit of reverse transmission in a conventional OFDM system. The portion designated by reference numeral 310 in FIG. 3 includes 16 cells. That is, 8 consecutive sub-carriers are distributed over two OFDM symbols, which indicates that the portion 310 has a structure which can carry the outputs of the 16 point DFT unit. In the prior art, there is a one-to-one mapping relation between the forward resource channels and the DFT inputs. In other words, the downlink resource channels 0 to 7 are mapped to the DFT inputs 0 to 7, which are then loaded on the portion designated by reference numeral 310. In the same manner, the downlink resource channels 8 to 15 are mapped to the DFT inputs 0 to 7, which are then loaded on the portion designated by reference numeral 320. Also, in the same manner, the downlink resource channels 16 to 23 are mapped to the DFT inputs 0 to 7, which are then loaded on the portion designated by reference numeral 330. Moreover, in the same manner, the downlink resource channels 24 to 31 are mapped to the DFT inputs 0 to 7, which are then loaded on the portion designated by reference numeral 330. As described above, portions corresponding to one-half of the tile shown in FIG. 3 are used for transmission of the reverse ACK/NACK. Each of the portions designated by reference numerals 310, 320, 330 and 340 are usually called a “sub-tile.” Further, three more tiles each having the same structure as that shown in FIG. 3 are used for the transmission. As a result, a total of four tiles each having the same structure as that shown in FIG. 3 are used for transmission of the reverse ACK/NACK. The four tiles have a simply repeated structure and are spaced from each other. The reason why the structure is simply repeated four times is in order to enhance the reliability in receiving the transmitted ACK/NACK. In brief, a total of 16 sub-tiles are used the reverse ACK/NACK transmission, which indicates that resources corresponding to two tiles from among a total of 30 reverse tiles are used for the reverse ACK/NACK transmission. In the above-described structure, the inputs of DFT 8 to 15 are not used so that they can be used for measurement of interference to the tile by a base station receiver. The four sub-tiles transmitting one ACK/NACK bit as described above experience different interferences. Therefore, the receiver measures the quantity of interference for each sub-tile during the process of demodulating the single ACK/NACK bit repeatedly received four times over the four sub-tiles, and uses different weights according to the quantity of interference in combining the four-time-repeated ACK/NACK, thereby improving the reception capability.
Meanwhile, when a plurality of layers are used for forward data transmission, a conventional method for reverse ACK/NACK transmission and resource allocation for the transmission correspond to extension of the method described above with reference to FIGS. 2 and 3 by an amount corresponding to the number of the layers. For example, when two layers are used in the forward direction, a total of 16 sub-tiles are used for the reverse ACK/NACK transmission. When four layers are used in the forward direction, a total of 32 sub-tiles are used for the reverse ACK/NACK transmission. In other words, four tiles and eight tiles are used for the reverse ACK/NACK transmission, respectively. This implies that 13.3% and 26.7% of the entire resources are used for the reverse ACK/NACK transmission, respectively. That is, too many resources are used for the reverse ACK/NACK transmission.
Meanwhile, in order to perform data transmission for multiple layers and support HARQ for each layer, it is necessary to consider an efficient ACKCH. Hereinafter, a conventional method for forward ACKCH transmission in a system performing reverse data transmission for multiple layers and supporting HARQ for each layer will be described.
First, a transmission method and a resource allocation method for a forward ACKCH for one layer in a conventional reverse OFDMA system are discussed. In a typical forward OFDMA system, a basic resource unit for forward data transmission is defined. As used herein, the basic resource unit is referred to as a “data resource channel.” Usually, one data resource channel is defined for multiple OFDM symbols in the time domain and for multiple sub-carriers in the frequency domain. For example, one data channel includes 8 OFDM symbols and 16 sub-carriers. If a system includes, for example, 480 sub-carriers available in the frequency domain, and if the system includes 30 data resource channels, a maximum of 30 bits are necessary for reverse ACK/NACK transmission. This is because it should be possible to transmit one bit of reverse ACK/NACK feedback for each forward data resource channel.
That is, transmission of the same number of reverse ACK/NACK responses as the number of forward data resource channels should be possible. Hereinafter, specific examples of the forward ACK/NACK transmission and resource allocation for the forward ACK/NACK transmission according to the prior art will be discussed.
FIG. 4 illustrates a conventional structure of a transmitter transmitting a forward ACK/NACK response to reverse data transmission from a plurality of MSs.
The conventional transmitter includes a 3-time repeater 402, an interleaver 403, a sub-carrier mapper 404, a multiplexer 405, an Inverse Fast Fourier Transform (IFFT) unit 406, a Parallel-to-Serial (P/S) converter 407 and a CP adder 408.
In FIG. 4, reference numeral 401 denotes an ACK/NACK bit stream. That is, one ACK/NACK bit is transmitted to one MS. The ACK/NACK has a value that is determined according to whether the received forward data has been correctly demodulated or has not been correctly demodulated and requires retransmission thereof. The 3-time repeater 402 repeats the ACK/NACK three times. Since it is assumed that there are 30 reverse resource channels, the 3-time repeater 402 yields 90 bits of outputs. Then, the interleaver 403 interleaves the 90 bits of outputs according to an interleaving scheme and then sends the interleaved outputs to the sub-carrier mapper 404. Then, the sub-carrier mapper 404 maps the 90 bits to sub-carriers at locations in the time and frequency domain from among the 480 sub-carriers. That is, 90 bits are transmitted by multiple sub-carriers and multiple OFDM symbols. Usually, in the mapping, the 90 bits are distributed as widely as possible in the time and frequency domain, in order to achieve the diversity effect as much as possible.
The multiplexer 405 multiplexes the outputs of the sub-carrier mapper 404. Specifically, the multiplexer 405 multiplexes the outputs of the sub-carrier mapper 404 with another channel, for example, a forward data channel. The output of the multiplexer 405 is transmitted after being processed by the IFFT unit 406, the P/S converter 407 and the CP adder 408 according to a typical OFDM symbol generating process.
Meanwhile, when a plurality of layers are used for reverse data transmission, a conventional method for forward ACK/NACK transmission and resource allocation for the transmission correspond to extension of the method described above with reference to FIG. 4 by an amount corresponding to the number of the layers. That is, the input ACK/NACK bit stream of FIG. 4 has a size of the number of reverse resource channels×the number of layers. For example, when two layers are used in the reverse direction, the forward ACK/NACK bit stream has a size of 60 (30×2). When four layers are used in the reverse direction, the forward ACK/NACK bit stream has a size of 120 (30×4).
The bit stream is repeated three times as described above, and the other process is the same as in FIG. 4. According to the method as described above, even when reverse transmission for multiple layers is supported, the number of resource channels of the multiple layers is actually limited. However, the method uses as many forward resources as a multiple of the number of the layers, so as to cause a waste of forward resources.